People that have two different sets of DNA are called human chimeras. It can happen when a woman is pregnant with fraternal twins and one embryo dies very early on. The other embryo can "absorb" its twin's cells. It can also happen after a bone marrow transplant, and (in a smaller scale) during normal pregnancy.

What is the purpose of chimeras?

Chimeras are incredibly useful for understanding how animals grow and develop. They might one day be used to grow life-saving organs that can be transplanted into humans.

Can a human be a chimera?

Human chimeras were first discovered with the advent of blood typing when it was found that some people had more than one blood type. Most of them proved to be "blood chimeras" -- non-identical twins who shared a blood supply in the uterus. ... About 8% of non-identical twin pairs are chimeras.